Communications and data transmission systems that transmit information signals in the form of optical pulses over optical fiber are now commonplace, and optical fibers have become the physical transport medium of choice in long distance telephone, data and video communication networks due to their signal transmission capabilities, which greatly exceed those of mechanical conductors. Despite their advantages, however, difficulties in their manufacture must be overcome in order for lengthy, high-yield and error-free optical fiber to be produced in mass.
The manufacture of optical fiber utilizes a glass preform from which optical fiber is generated. The glass preform reproduces the desired index profile of the optical fiber in a thick glass rod. After a preform is created, it is loaded into a fiber drawing tower. The lower end of the preform is lowered into a furnace so that the end of the preform is softened until a softened glob falls down by gravity. As it falls, it forms a thread. The thread cools as it falls, and undergoes a series of processing steps (e.g., application of coating layers) to form the finished optical fiber. Therefore, it will be appreciated that the make-up and length of optical fiber generated by this process is dependent upon the characteristics of the preform from which the optical fiber is drawn.
The basic manufacturing steps of generating preforms are well known to those of skill in the art. Three basic forms for the production of preforms include: Internal Deposition, where material is grown inside a tube; Outside Deposition, where deposition is done on a mandrel removed in a later stage; and Axial Deposition, where deposition is done axially, directly on the glass preform. One of the most common and widely-used processes in optical fiber preform production is Modified Chemical Vapor Deposition (MCVD), which is a type of Internal Deposition. MCVD is a process for fabricating preforms wherein preform core material is deposited on the inside surface of a substrate or starting tube (‘substrate tube’ and ‘starting tube’ are used interchangeably herein). Individual layers of deposited material are turned into glass (vitrified) by a torch that moves back and forth along the length of the tube. During a deposition process the torch assembly slowly traverses the length of the starting tube while reactant gasses are fed into and exhausted from the tube. Following the deposition of core material and/or cladding material, the starting tube is collapsed to form a solid core rod by heating it to a higher temperature than during deposition. After the core rod is generated, during an overcladding process material such as silica is added to increase the diameter of the core rod. After overcladding, the optical fiber perform is complete and ready to be drawn into optical fiber.
Although the generation of preforms by the method described above are commonly utilized in optical fiber manufacturing, preforms generated by this process often suffer from ovality; that is, the preforms do not necessarily have a circular cross section throughout their entire length. Preform ovality is undesirable because it changes and more often increases the Polarization Mode Dispersion (PMD) of optical fiber. PMD is a stochastic phenomenon that leads to the dispersion of the optical pulses transmitted in an optical fiber. In particular, the dispersion is caused by the propagation speed difference between the polarization modes of the fiber. PMD limits the transmission capacity of optical communication systems by creating inter-symbol interference. Because low PMD is a desirable characteristic of optical fiber, reducing preform ovality is a crucial factor in achieving desirable transmission characteristics of optical fiber.
Therefore, what is needed is a method for achieving desired preform core ovality to reduce the PMD of an optical fiber generated there from.